The long range objective of this program is to test the following detailed hypothesis for the maturation of B lymphocytes in rats: Slowly dividing pleuripotential stem cells found in the bone marrow and spleen give rise to progenitor B cells. The turnover rate of the latter cells is regulated by a feedback mechanism related to the concentration of more mature B cells in the peripheral lymphoid tissues. The progenitor B cell gives rise to a rapidly dividing generator B cell found in the spleen and the marrow. The latter cell rapidly and continuously populates the peripheral lymphoid tissues with short-lived, nonrecirculating virgin B cells. Upon interaction with antigen the virgin B cell gives rise to antibody forming cells and long-lived, recirculating memory B cells. Immunoglobulins, receptors for complement, and specific antigen-binding receptors are found on the surface of B generator cells and their progeny. We will test this hypothesis by determining the tissue distribution, migration pattern, turnover rate, cell surface characteristics, and ancestry of B cells and B cell precursors found in the marrow, spleen, and thoracic duct lymph of rats. We expect to separate the various cells described in this maturation scheme by lg velocity sedimentation. Immunological function will be examined in cell transfer experiments.